SAN FRANCISCO (Reuters) - Spam has hit the Kindle, clogging the online bookstore of the top-selling eReader with material that is far from being book worthy and threatening to undermine Amazon.com Inc's publishing foray.

Thousands of digital books, called ebooks, are being published through Amazon's self-publishing system each month. Many are not written in the traditional sense.

Instead, they are built using something known as Private Label Rights, or PLR content, which is information that can be bought very cheaply online then reformatted into a digital book.

These ebooks are listed for sale -- often at 99 cents -- alongside more traditional books on Amazon's website, forcing readers to plow through many more titles to find what they want.

Aspiring spammers can even buy a DVD box set called Autopilot Kindle Cash that claims to teach people how to publish 10 to 20 new Kindle books a day without writing a word.

This new phenomenon represents the dark side of an online revolution that's turning the traditional publishing industry on its head by giving authors new ways to access readers directly.

BY THE NUMBERS

In 2010, almost 2.8 million nontraditional books, including ebooks, were published in the United States, while just more than 316,000 traditional books came out. That compares with 1.33 million nontraditional books and 302,000 conventional books in 2009, according to Albert Greco, a publishing-industry expert at Fordham University's business school.

In 2002, fewer than 33,000 nontraditional books were published, while over 215,000 traditional books came out in the United States, Greco noted.

"This is a staggering increase. It's mind boggling," Greco said. "On the positive side, this is helping an awful lot of people who wrote books and could not get them published in the traditional way through agents," Greco added.

But Greco listed downsides. One problem is that authors must compete for readers with a lot more books -- many of which "probably never should have seen the light of day," he said.

Some of these books appear to be outright copies of other work. Earlier this year, Shayne Parkinson, a New Zealander who writes historical novels, discovered her debut "Sentence of Marriage" was on sale on Amazon under another author's name.

The issue was initially spotted and then resolved by customers through Amazon's British online forum.

"How did I feel? Shocked and somewhat incredulous, but at the same time, because of the way I found out, very grateful that someone had taken the trouble to let me know," Parkinson said.

For Amazon, the wave of ebook spam crashing over the Kindle could undermine its push into self-publishing and tarnish the brand of the best-selling Kindle eReader, which is set to account for some 10 percent of the company's 2012 revenue, according to Barclays Capital estimates.

"It's getting to be a more widespread problem," said Susan Daffron, president of Logical Expressions, a book and software publishing company. "Once a few spammers find a new outlet like this, hoards of them follow."

Amazon pays authors 70 percent to 35 percent of revenue for ebooks, depending on the price. That gives spammers a financial incentive to focus on this new outlet.

"Amazon will definitely have to do more quality control, unless they want the integrity of their products to drop," she added.

"Amazon will work hard to snuff this out as it undermines many of its advantages in the space," said James McQuivey, an eReader analyst at Forrester Research.

Amazon is curating submissions to its new Kindle Singles business, which offers short stories, long-form journalism and opinion pieces, "after seeing how quickly the self-published side degenerated," McQuivey noted.

"Undifferentiated or barely differentiated versions of the same book don't improve the customer experience," Amazon spokeswoman Sarah Gelman wrote in a June 14 email to Reuters. "We have processes to detect and remove undifferentiated versions of books with the goal of eliminating such content from our store." She did not respond further.

DO-IT-YOURSELF SPAM

Kindle spam has been growing fast in the last six months because several online courses and, ironically, ebooks have been released that teach people how to create a Kindle book per day, according to Paul Wolfe, an Internet marketing specialist.

One tactic involves copying an ebook that has started selling well and republishing it with new titles and covers to appeal to a slightly different demographic, Wolfe explained.

Spam has yet to flood the online bookstore of the Nook, a rival eReader sold by Barnes & Noble Inc.

The company may be managing ebook submissions more aggressively than Amazon, but it might just be that the Kindle's huge audience is more attractive to spammers, Forrester's McQuivey said. Barnes & Noble did not respond to requests for a comment.

Smashwords, an ebook publisher and distributor, has also struggled with spam, but not to the same degree as Amazon's Kindle, according to Founder Mark Coker.

Smashwords, which competes with Amazon, manually checks the formatting and other basic characteristics of the submissions it receives, before publishing. Obvious signs of spam include poorly designed covers, the lack of an author's name on the cover and bad formatting, Coker explained.

Smashwords pays authors quarterly, while Amazon pays monthly, Coker added. The longer payment period means Smashwords has more time to track down spammers and close accounts before money changes hands, he said.

Amazon does not offer many free ebooks, while Smashwords does. So there is more of an incentive to publish lots of books via the Kindle, according to Coker.

Coker said his company has found five or six instances when free ebooks published on Smashwords were copied and republished on Amazon's Kindle store for at least 99 cents each.

Forrester's McQuivey said Amazon will have to craft a social-network solution to the problem. If the company can let readers see book recommendations from people they know, or people whose reviews they liked in the past, that would help them track down the content they want and avoid misleading recommendations, he explained.

Daffron of Logical Expressions said Amazon should charge for uploads to the Kindle publishing system because that would remove a lot of the financial incentive for spammers.

"This is why email spam has become such a problem -- it costs nothing," she said. "If people can put out 12 versions of a single book under different titles and authors, and at different prices, even if they sell just one or two books, they can make money. They win and the loser is Amazon."